From AMS Glossary
(Also called olefins.) Monoalkenes are hydrocarbon species with general formula CnH2n, where n is an integer, containing one unsaturated carbon–carbon bond.
Alkenes are emitted in large quantities by automobiles and by vegetation. They react moderately to very rapidly with hydroxyl radicals and with ozone, which makes them major contributors to atmospheric reactivity, but limits their concentrations in the atmosphere. More complex dialkenes, with more than one double bond, can also be formed.